1. Field of the Invention
This invention relates to an improved method for horizontal ribbon crystal growth and more particularly relates to a method for preventing recrystallization which is apt to occur in the melt of a crystalline substance upon contact with the upper peripheral wall of a melt crucible.
2. Description of the Prior Art
There is known a method for conducting a horizontal ribbon crystal growth wherein crystalline substance, for instance semiconductor silicon or the like is heated in a refractory crucible by means of a heater to make melt. The melt is raised up to the level beyond the upper edge of the crucible and from the horizontal free surface of said melt a sheet-like crystal is grown at the growing tip of a seed crystal. The crystal is drawn out in a horizontal direction.
In such a method for crystal growth, there are two technical problems. One of the problems is to enhance the cooling effect, when the contact intersurface (hereinafter referred to as the solid-liquid interface) between the seed crystal or the successively growing crystal and the melt is cooled by cooling gas from a cooler arranged above said interface. The other problem is to prevent recrystallization, which is apt to be caused on the upper surface of the melt contacting with the upper portion of a peripheral wall of a crucible and where heat radiation takes place in large quantity.
Should the upper end edge of the crucible be positioned lower than the upper surface of the melt, the desired cooling effect problem will be attained. However, such cooling induces the generation of recrystallization at a position adjacent to the upper portion of the peripheral wall of the crucible.
Should a heater for heating the upper portion of the outer peripheral wall of the crucible be arranged outside or inside of the peripheral wall, recrystallization can be prevented. However the cooling effect for the solid-liquid interface is reduced.
FIG. 1 is a vertical sectional view schematically showing a state when a ribbon crystal is being drawn out according to the prior art. Surface of melt 1 of high purity silicon is held to a level raised up from the upper edge of a high purity quartz crucible 4. On the horizontal free surface 1a of this melt 1, a seed crystal 3 is contacted with melt to adapt thereto. Thereafter by increasing the flow rate of gas of a cooler 8 arranged above the crystal growth part, the cooling of the solid-liquid interface between the seed crystal 3 and the melt 1 is intensified so as to become undercooled. Thereby a ribbon crystal 2 is grown and horizontally drawn out in the direction opposite to the direction of crystal growth by means of a drawing mechanism (not shown) pulling the seed crystal 3 with the same velocity as its growing rate.
In the method shown in FIG. 1, a heater 7 surrounds almost the whole periphery of a peripheral wall 5a of a crucible 5 made of high purity graphite holding said crucible 4 and covers only the peripheral portion of a bottom portion 5b of the crucible 5 and does not heat the central portion of the bottom portion 5b. And, the upper end edge of this heater 7 is positioned lower than the upper surface 1a of the melt 1.
Above this heater 7, another heater 6 is arranged, and 9 is a heat shield plate.
In this arrangement, when electric power is supplied to the heater 7 only, the cooling effect for the solid-liquid interface can be enhanced because the heating for the upper portion of the melt is weak. The circulating convection caused in the melt 1 within the crucible 4 is shown by broken lines in FIG. 1. Namely, the melt 1 is heated by the heater 7 at a place near the peripheral wall 4a of the crucible 4 and its density becomes smaller, so that it ascends along peripheral wall 4a. Thereafter it flows along the upper surface 1a of the melt 1 towards the central portion, and in this course it flows underside of the solid-liquid intersurface, where it is cooled by cooling gas to increase its density so that it descends and reaches the bottom portion 4b of the crucible 4. Thereafter it flows from the central portion of this bottom portion towards the peripheral portion.
Due to the flow of the melt underside of the solid-liquid interface, the crystal growth in depth direction of the melt can be prevented. However, as mentioned above, in case electric power is supplied to the heater 7 only, the recrystallization of melt 1 occurs at a place adjacent to the upper portion of the peripheral wall 4a.
On the other hand, when electric power is supplied to the heater 6 as well as to heater 7, recrystallization can be prevented. However, the cooling effect for the solid-liquid interface is weakened. Further, in this case, the circulating melt as shown by the broken lines will not occur, because the temperature of the melt contacting with the upper portion of the peripheral wall 4a of the crucible becomes higher than the temperature of the melt contacting with the lower portion of the peripheral wall 4a and the density of the melt of this upper portion becomes less than that of the lower portion. Thus any force to upwardly drive the melt along the peripheral wall 4a will not occur. Consequently, crystal growth in the depth direction of the melt will occur.